Many signals of interest have bandwidths that are much larger than the bandwidth of the test equipment utilized to measure the signals. For example, the bandwidth of a conventional oscilloscope is typically less than that of radio frequency (RF) signals used in communication systems and the like. Hence, measuring such signals on an oscilloscope presents a problem. A sampling oscilloscope circumvents this problem for repetitive signals by utilizing a sampling circuit that measures the signal over a very brief time interval and displays the resulting sample as one point of a graph. Typically, one sample is taken during each period of the repetitive signal. The time of the sample relative to the beginning of the signal repetition is varied in each period such that successive points sample the signal at different points relative to the beginning of each period of the signal. Hence, the collection of samples can be displayed to provide a conventional display of voltage as a function of time.
Only a small fraction of the energy in the signal is extracted at each sample. The amount of energy that is extracted depends on the time interval over which the sampling window is opened. The sampling window must be of sufficiently short duration that changes in the signal amplitude during the sampling interval can be ignored. Hence, as the bandwidth of the signal being measured increases, the sampling time interval must be decreased, leading to still further decreases in the amount of energy that is extracted from the signal. The ratio of this sample energy to the noise in the instrument determines the signal-to-noise ratio of the instrument. Higher signal-to-noise ratios provide more accurate measurements, and hence, are preferred.
In order to increase the signal-to-noise ratio of the sampled system, the repetition rate of the sampling may be increased. However, an increase in the sampling rate is not always practical, because the electronics following the sampler also have bandwidth limitations. Also, the amount of energy extracted from the signal may be improved by utilizing a mixer in which a sinusoidal local oscillator signal is used to extract energy from the signal. However, this approach is only practical for signals having limited bandwidth, such as a modulated RF carrier in which the carrier is removed, so that the modulation waveform can be examined.
In addition, test instruments, such as network analyzers and oscilloscopes, are designed to measure various properties of a device under test (DUT) in response to stimulus signals. Such properties include frequency response parameters of the DUT, including S-parameters, as well as reflectivity, group delay, dispersion, impulse response, nonlinear characteristics, electrical impedance, etc. Typically, test instruments have at least two ports to enable transmission measurements of the DUT, such as S21 and S12 measurements, where a first port receives the stimulus signal and a second port receives an input signal provided by the DUT responsive to the stimulus signal, so that the test instrument is able to compare the signals. However, transmission measurements may be difficult to obtain using a single test instrument when the DUT is relatively long and/or is difficult to access via both ports of the test instrument.
For example, it would be difficult to measure S21 and S12 of a 100 m long coaxial cable (the DUT) installed in a submarine. The length of the cable and its attenuation prevent accurate measurement of S21 by simply measuring the reflection off of the far end of the cable (as is sometimes done). With one test instrument, S21 may be measured by connecting yet another very long cable to the far end of the DUT and to the second port on the test instrument. However, this additional cable also has losses, which reduce the dynamic range of the measurement. Also, the effort expended in running this extra length of cable would make the measurement complicated and difficult to perform.
Alternatively, two network analyzers may be used to make the S21 measurement. Careful synchronization between the two network analyzers would enable measurement of the amplitude of S21, but not the phase. Phase cannot be measured because the phase of the transmitted signal would certainly vary in an unpredictable way relative to the phase of the receiver's local oscillator (LO) as the signal and LO are tuned, thus rendering the phase of the measurement ambiguous. Also, enabling the synchronization required for even the amplitude measurement typically involves a cable run between the two network analyzers, which makes this approach no more efficient than the previously discussed approach.
Channel sounding is another example of an application performed by test instruments. Channel sounding measurements are intended to characterize a signal path, including the S-parameters, between two or more antennas that may be located miles apart, where the signal path is considered the DUT. In multiple-input multiple-output (MIMO) applications, for example, multiple antennas are employed for both the transmitter and receiver ends of the DUT. The S-parameters of the “channel” between each pair of antennas is measured for MIMO channel sounding. Running cables between transmitter and receiver antennas is impractical for such long channels of the MIMO antenna arrays. Conventionally, S-parameter measurements may be recovered using large and expensive racks of instruments, one instrument rack for the transmitter and another instrument rack for the receiver. The transmitter and receiver instrument racks must be synchronized, e.g., using corresponding atomic clocks. Accordingly, making S-parameter measurements of long channels, e.g., defined by antennas widely separated in space, is expensive, complicated and time-consuming. Further, for high frequency signal, in particular, a high-speed oscilloscope (e.g., greater than 6 GHz) may be used to capture the receiver's data with sufficient bandwidth. However, such high-speed oscilloscopes are very expensive, costing tens of thousands of dollars.